This invention relates to an improved intrusion alarm system and especially to an alarm system which gives alarms when it detects a trespasser and informs a security center.
An alarm system which detects a trespasser by detecting door opening and closing, when a resident is not in a house, office, room or the like, and which gives an alarm, has been proposed. In this type of alarm system, an alarm signal is generated when a door is opened by someone while the resident is not in the house. For this purpose, a mode selector provided on the control panel of the alarm system is set to the "Absent" mode. However, if the detection is made immediately after the mode setting, the door opening by the resident who is going out might be erroneously detected as the opening by a trespasser.
Therefore, the door opening detection is not performed during a predetermined time interval (e.g. 1 minute) to allow time between mode setting and the final door closing. However, if the resident does not go out after setting of the "Absent" mode within the above mentioned predetermined time, the door opening and closing needed for going out will be detected and the resident will be misconceived as a trespasser. In this type of alarm system, when the trespasser is detected, an alarm signal representing abnormal state is transmitted to a security center, and there will be serious confusion.